Kinetic study of the rotator phase transition in an n-alkane (n-C25H52) crystal: Nucleation and growth on cooling

The kinetics in the solid-solid phase transition of an n-alkane crystal, which is the so-called 'rotator phase transition', is investigated by means of differential scanning calorimetry (DSC), X-ray diffraction (XRD) and optical microscopy. Primary nucleation rate in the transition on cooling is measured by using droplet crystals, while growth rate is measured by using a single crystal, independently. The growth rate increases rapidly with the increase in the degree of supercooling compared with that on heating. A significant asymmetric behavior of the temperature dependence of the growth rate is observed between temperatures above and below the transition point. The kinetic parameters, i.e., the surface free energies between two solid phases, are estimated from a combination of the primary nucleation rate and the growth rate. It is found that the primary nucleation in the rotator phase transition of n-alkane droplet on cooling is a homogeneous one. Furthermore, the surface free energies on cooling are slightly smaller than those on heating, and this seems to induce the asymmetric behavior of the growth rate.